P0304
Cylinder 4 Misfire Detected
Ignition / misfire
Generic OBD-II
High
What it means
P0304 means the engine's misfire monitor has detected that cylinder 4 specifically is not firing reliably — fuel isn't burning properly in that cylinder, so the engine runs rough and loses power. Because it's isolated to one cylinder, it usually points to that cylinder's spark plug, coil, or injector. Sustained misfires can overheat and ruin the catalytic converter.
High
Can I keep driving? Limit driving — a misfire on cylinder 4 dumps raw fuel into the exhaust and can overheat the catalytic converter, especially if the light is flashing.
Common symptoms
What you'll notice
- Check-engine light on (flashing during an active misfire)
- Rough idle and noticeable shaking or vibration
- Reduced power and hesitation on acceleration
- Increased fuel consumption and a possible fuel smell
Common causes
Most likely first
- Worn or fouled spark plug on cylinder 4Likely
- Failing ignition coil on cylinder 4Common
- Clogged or leaking fuel injector on cylinder 4Common
- Vacuum leak near cylinder 4's intake runnerLess often
- Low compression on cylinder 4 (valve, ring, or head-gasket issue)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data, then swap the cylinder-4 coil and plug with a neighbouring cylinder and re-scan — if the misfire follows the part, you've found it.
- Inspect and replace the spark plug and coil on cylinder 4 first — the most common cause.
- Test the cylinder-4 injector (resistance and spray) and check fuel delivery.
- Check for a vacuum leak around that runner with carb cleaner or smoke.
- If it persists, run a compression/leak-down test on cylinder 4 for mechanical faults, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0304?+
Only briefly. A single-cylinder misfire sends unburned fuel into the catalytic converter and can destroy it — if the check-engine light is flashing, drive as little as possible.
Can I just clear P0304?+
You can, but it returns within a few drive cycles until the cause on cylinder 4 is repaired. Fixing the plug, coil, or injector is what clears it for good.
Related codes
Often seen with P0304
Read & clear P0304 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX